fix(requirements): raise InvalidRequirement for malformed specifier lists

[r266-tech]

Summary

• translate SpecifierSet validation failures into InvalidRequirement, matching Requirement's documented exception contract
• cover both direct construction and stable-pickle restoration with the malformed demo===x,y form

Why

The requirement parser accepts the arbitrary-equality token and comma, but the later SpecifierSet construction rejects the bare y. That currently leaks InvalidSpecifier from Requirement(...), so callers cannot consistently handle malformed requirement strings via InvalidRequirement; the same leak also bypasses Requirement.__setstate__'s invalid-string conversion.

Validation

• uv run pytest -q — 62,115 passed, 1 platform skip
• uv run pytest tests/test_requirements.py -q — 5,322 passed
• uv run ruff check src/packaging/requirements.py tests/test_requirements.py
• uv run ruff format --check src/packaging/requirements.py tests/test_requirements.py